1. Field of the Invention
The present invention relates to a density signal determination method of determining the density values of a latent image and a background image, which constitute a copy-forgery-inhibited pattern image, and to an image forming apparatus and an image processing system, which are configured to form on a sheet a copy-forgery-inhibited pattern image whose density value is determined.
2. Description of the Related Art
There are special sheets called forgery-inhibited sheets or security paper. In a forgery-inhibited sheet, a string of characters, such as “COPY”, is hid so as to be invisible on a casual glance. However, the hid string of characters appears on a copy obtained by copying the forgery-inhibited sheet. Thus, a document produced by using such a forgery-inhibited sheet can easily be discerned from the copy. Also, hesitation about using a copy of the document can be caused.
Forgery-inhibited sheets have the above advantages. Therefore, forgery-inhibited sheets have been utilized to make ledger sheets, resident cards, and so on. However, forgery-inhibited sheets have drawbacks in that they are high in price, as compared with sheets of plain paper, and that only strings of characters hid in the forgery-inhibited sheet at production thereof can appear on a copy thereof.
Under such situations, in recent years, a new technique capable of obtaining advantages similar to those of forgery-inhibited sheets has attracted attention (see Japanese Patent Application Laid-Open No. 2001-197297). This technique of synthesizing pattern-containing-image data from original data and copy-forgery-inhibited pattern (also referred to as copy-inhibited pattern) image data in a printer and of outputting the pattern-containing-image data, which is obtained by this synthesis, to a sheet of standard paper. Incidentally, a string of characters, or the like is hid in this copy-forgery-inhibited pattern image. Thus, the hid string of characters appears on a copy obtained by copying the pattern-containing-image, similarly to the case of using a forgery-inhibited sheet. Additionally, this technique utilizes a sheet of standard paper. Therefore, this technique has an advantage in that an original can be produced inexpensively, as compared with the case of utilizing a forgery-inhibited sheet. Also, according to this technique, new copy-forgery-inhibited pattern image data can be generated every time an original is produced. Thus, this technique has another advantage in that the color of a copy-forgery-inhibited pattern image and a hid string of characters can be freely set.
Meanwhile, this copy-forgery-inhibited pattern image includes an area in which an image “remains (appears)” on the copy, and an area in which an image “disappears” on the copy (or the density of this area is low, as compared with that of the former area). Incidentally, these two areas have substantially the same reflection density on the original. Thus, the human eye cannot recognize that a string of characters, such as “COPY”, is hid in the original. Incidentally, the word “remain” means that an image on an original is exactly reproduced on a copy. Further, the word “disappear” means that an image on an original is not reproduced on a copy. Additionally, the reflection density is measured by a reflection densitometer.
Hereinafter, the area, in which an image “remains” on the copy, is referred to as a “latent image part”. The area, in which an image “disappears” on the copy (or the density of this area is low, as compared with that of the former area), is referred to as a “background part”.
FIG. 38 is a view showing the state of dots in a copy-forgery-inhibited pattern image. In FIG. 38, an area in which dots are arranged in a concentrated manner is a latent image part. An area in which dots are arranged in a dispersed manner is a background part. Dots placed in each of the two areas are formed by an associated one of different halftone dot processes or different dithering processes. For example, the dots of the latent image part are formed by a halftone dot process using a small number of lines. The dots of the background part are formed by a halftone dot process using a large number of lines. Alternatively, the dots of the latent image part are formed by using a dot-concentrated-type dither matrix. The dots of the background part are formed by using a dot-dispersed-type dither matrix.
Meanwhile, the reproduction ability of a copying machine depends on the input resolution and the output resolution of the copying machine. Thus, there is a limit to the reproduction ability of the copying machine. Consequently, in the case where the dots of the latent image part of the copy-forgery-inhibited pattern image are formed to be larger than dots that can be reproduced by a copying machine, and where the dots of the background part of the copy-forgery-inhibited pattern image are formed to be smaller than dots that can be reproduced by the copying machine, generally, the dots of the latent image part on a copy are reproduced, while the dots of the background part are difficult to reproduce. As a result, the latent image part is densely reproduced on the copy in comparison with the background part. Hereinafter, a phenomenon in which a hid string of characters appears on a copy because the latent image part is densely reproduced in comparison with the background part is called “visualization”.
FIGS. 39A and 39B are views illustrating this “visualization”. FIGS. 39A and 39B conceptually illustrate that concentrated dots (large dots) are reproduced on a copy, and that dispersed dots (small dots) are not exactly reproduced thereon.
Incidentally, the configuration of the copy-forgery-inhibited pattern image is not limited to the aforementioned configuration. It is sufficient to configure the copy-forgery-inhibited pattern image so that human recognizable strings of characters, such as “COPY”, symbols, or patterns appear (or are visualized) on a copy. Even in the case where the string of white characters, such as “COPY”, on a colored area is shown, this copy-forgery-inhibited pattern image substantially achieves the purpose thereof. In this case, the region of the characters “COPY” is called the “background part”.
Meanwhile, as described above, the copy-forgery-inhibited pattern image is formed so that the reflection densities of the latent image part and the background part are substantially equal to each other. For instance, Japanese Patent Application Laid-Open No. 2001-197297 describes that tone correction is performed to make the tone of the latent image part having undergone a halftone dot process substantially equal to that of the background part not having undergone the halftone dot process when being outputted.
Although not describing copy-forgery-inhibited patterns, Japanese Patent No. 3235926 and Japanese Patent Application Laid-Open No. 2000-196879 describe techniques for adjusting the light exposure and a PWM (Pulse Width Modulation) parameter so as to form an image of a small-dot area having a reflection density substantially equal to the reflection density of a large-dot area.
However, the size of dots outputted by an apparatus, for example, MFP (Multi-Function Printer) or a printer, varies under the influence of the environment in which the apparatus is disposed, and the number of years of use of the apparatus. Consequently, the reflection density of the output image is changed. Thus, even in the case where initial setting is performed by a method described in Japanese Patent Application Laid-Open No. 2001-197297 so that the reflection densities of the latent image part and the background part are substantially equal to each other, when the size of outputted small dots is slightly larger (or smaller) than that of small dots outputted the last time, a copy-forgery-inhibited pattern image, in which a background part is darker (or lighter) than a latent image part, is formed. As a result, there is a problem in that the latent image is visible on an original (that is, the concealability of a latent image is low).
Also, when the size of outputted large dots is increased (or decreased), a copy-forgery-inhibited pattern image, in which a latent image part is darker (or lighter) than a latent image part, is formed. As a result, there is the problem in that the latent image is visible on an original.
Also, in the case where both the size of large dots and that of small dots are increased, an original having a darker copy-forgery-inhibited pattern image is produced. Consequently, there are problems in that the legibility of contents, for example, a full name written on the resident card, is lowered, that both of the latent image part and the background part are reproduced on a copy, and that the latent image does not appear on the copy. Conversely, in the case where both the size of large dots and that of small dots are decreased, an original having a lighter copy-forgery-inhibited pattern image is produced. Consequently, there are problems in that neither the latent image part nor the background part is reproduced on a copy, and that the latent image does not appear on the copy.
To summarize the foregoing, there are problems in that a copy-forgery-inhibited pattern image having low concealability of a latent image is formed on an original, that a copy-forgery-inhibited pattern image having low legibility of contents is formed thereon, and that a copy-forgery-inhibited pattern image which is not visualized on a copy is formed thereon.
Meanwhile, Japanese Patent No. 3235926 and Japanese Patent Application Laid-Open No. 2000-196879 disclose the techniques of outputting a test sheet having a plurality of patches and subsequently causing a user to determine patches which appear to have a density equal to that of a predetermined patch. According to the technique disclosed in Japanese Patent No. 3235926, the density is adjusted by determining an optimal exposure based on the result of the user's determination. According to the technique disclosed in Japanese Patent Application Laid-Open No. 2000-196879, the density is adjusted by determining an optimal PWM parameter based on the result of the user's determination.
However, according to the techniques disclosed in Japanese Patent No. 3235926 and Japanese Patent Application Laid-Open No. 2000-196879, the adjustment is performed by a user while visually checking. Thus, exact results cannot be obtained. Also, the techniques impose loads on users and are not very convenient for users.